A number of different valves may need to be used on pneumatic conveying plant, and a wide variety of different valves are available in the market place. Rotary valves have been considered at length, and are ideal for controlling the feed of material into or out of a system at a controlled rate. There is, however, a requirement for many other types of valve, generally to be used for the purpose of isolating the flow. Many of these have been included on sketches of conveying systems in previous chapters and include, dis- charge valves, vent line valves and diverter valves.
A valve in a conveying line that is required to stop and start the flow is an onerous duty. Although the valve is only used in either the open or closed position, and is not used for flow control purposes, particulate material must be able to pass freely through when it is open. If the control surfaces of the valve remain in the flow path, as they will with pinch valves and ball valves, they must provide a perfectly smooth passage for the flow of material through the valve when open.
Any small protuberances or surface irregularities that could promote turbulence in the area would result in a rapid deterioration in performance. This is particularly the case when the material to be conveyed is abrasive. This type of valve is also very vulnerable during the opening and closing sequences, and so these operations should be completed as quickly as possible.
The author has tested numerous ball valves in a 100 mm bore pipeline conveying silica sand in dilute phase at 2 bar pressure. They did not perform very well in such a harsh environment. As they have moving parts the very fine abrasive dust in the conveyed material wrecked havoc. The valves soon lost their airtightness, and the torque required to operate the valves gradually increased and soon exceeded that available by the automatic control facilities provided with the valves.
Pinch valves are a much better proposition, as there is no relative movement between surfaces in which fine abrasive dust can lodge. These can also be opened and closed rapidly. Rubbers and urethanes also have very reasonable erosive wear resistance, and so are well worth considering for this kind of duty. They will not last forever, and so periodic maintenance is essential, and will be required. These valves must be located in an accessible position, and spares must be available.
The dome valve is a more recent addition to the list of valves available, but it has been specifically designed for this type of duty, and is now being widely used in the indus- try. The valve has moving parts, but these move completely out of the path of the conveyed material when the valve is open. On closing, the valve first cuts through the material and then becomes airtight by means of an inflatable seal. The valve can be water cooled and so it is capable of handling hot materials.